Carotid body control of ventilation will be studied using a previously developed technique which involves assessment of total carotid body neural output during progressive isocapnic hypoxia to evaluate hypoxic ventilatory drive. Possible alterations in carotid body sensitivity to hypoxia and hypercapnea due to changes in sympathetic input will be studied by stimulating the carotid branch of the superior cervical ganglion during steady state hypercapnea and progressive isocapnea. Possible effects of temperature on carotid body function will be determined by assessing hypoxic drive at body temperatures of from 32 to 42 degrees C. Past studies demonstrating occurance of ventilatory depression occuring at higher alveolar oxygen tensions as alveolar carbon dioxide tension is increased will be extended. Alveolar-arterial gas tensions will be evaluated to determine if end-tidal gas tensions provide accurate estimates of arterial gases under conditions of severe hypoxia and hypercapnea.